The striking similarities between the biochemical processes (eg, Na+-K+-ATPase activity, inositol metabolism, and intracellular organic solute metabolism) which are altered in insulin-dependent diabetes (IDD) and the mechanisms by which renal cells regulate their volumes (Na+-K+-ATPase-dependent, active ion transport processes regulated by cyclic AMP- and phosphoinositide- dependent events, and accumulation and depletion of organic "osmoregulatory" solutes) suggest that a fundamental defect in cell volume control may be present in IDD. Since cell volume homeostasis is vital for cell survival, this defect in cell volume regulation could initiate, or contribute to, diabetic renal cell injury. The long-term objective of the studies proposed in this grant is to evaluate the role of disturbances in cell volume regulation in the pathogenesis of tubular-interstitial damage and glomerular sclerosis in the streptozotocin-induced diabetic rat. Renal medullary cells would be particularly vulnerable to such a defect because they are frequently exposed to wide variations in their osmotic environment associated with normal urinary concentration and diluting functions. In addition, wide swings in extracellular glucose concentrations in IDD would osmotically stress glomerular mesangial cells (as well as other body cells). Studies will be performed in vitro using both isolated perfused medullary thick ascending limb (mTAL) and inner medullary collecting duct (IMCD) nephron segments and cultures of glomerular mesangial and inner medullary collecting duct cells. The volume regulatory responses of these cells to varying external osmolality will be evaluated by measuring the changes in (i) cell volume using optical, electrophysiological and isotopic techniques; (ii) cell ionic and organic solute contents (including glucose, sorbitol, and betaine); and (iii) the enzyme systems involved in ion and organic solute metabolism including Na+-K+-ATPase, aldose reductase, sorbitol dehydrogenase and choline oxidase. Specific protocols will assess the effects of variation in glucose control and duration of disease in streptozotocinduced diabetic rats and the "diabetic" environment (eg, high glucose, low insulin and myo- inositol) in cell culture systems on these cell volume regulatory parameters. Results of these studies could add importantly to our understanding of the pathogenic mechanisms of renal injury in IDD and suggest new forms of therapy or preventative measures.